Optically compensated bipolar transistor

ABSTRACT

A method for temperature compensation of a bipolar transistor through optically-induced carrier density enhancement. In response to the output of a temperature sensor, the optical output power of a photon source directed toward the bipolar transistor to be compensated is varied. Photons incident on the semiconductor surface effect variations in supplemental carrier concentration that maintain junction potential of the bipolar transistor at a predetermined level.

TECHNICAL FIELD

This invention relates generally to bipolar transistors, and inparticular to a method for temperature compensation of a bipolartransistor junction through optically-induced carrier densityenhancement.

BACKGROUND ART

The performance of bipolar transistors is strongly affected bytemperature variations. These variations in temperature manifestthemselves as a shift in the junction potential of the device. Astemperature decreases, junction potential increases rather dramatically.

Compensation schemes of the prior art, implemented externally to thedevice itself, simply increase or decrease input drive level to trackthe varying junction potential. This compensation method placesincreased demand on the stage being used to drive the bipolar transistorunder consideration, and adds cost and complexity to external circuitry.Also, at extremely low temperatures, it is possible that the device maynever turn on due to depletion of intrinsic carriers in the base region.

Accordingly, a need arises for a method for compensating a bipolartransistor for variations in junction potential caused by changes intemperature. The method should adequately compensate for decreases inintrinsic carrier concentration, but should not add undue complexity orcost to eventual circuit designs.

SUMMARY OF THE INVENTION

The above-described need may be addressed by placing a photon sourcewithin the transistor package. Bipolar transistors intended for radiofrequency (RF) amplifier applications are generally available inrelatively large packages. These packages can easily accommodate aphoton source and a temperature sensor. According to the invention, thetemperature of the bipolar transistor is sensed, and, in response tothis sensed temperature, a sufficient number of photons is directedtoward the bipolar transistor to substantially maintain the junctionpotential at a predetermined level. Temperature sensing is accomplishedthrough measuring junction potential of a reference diode located inclose proximity to the bipolar transistor, while the photon source is alight emitting diode.

In the preferred embodiment, the photon source has a variable outputpower, which is adjusted in response to temperature sensor output tosubstantially maintain the junction potential of the bipolar transistorat a predetermined level. The temperature sensor comprises a referencediode that is optically passivated; that is, subjected to additionalprocess steps known in the art to render it substantially impervious tophoton bombardment. In this way, the reference diode remains unaffectedby the photons, so that its junction potential accurately reflectstemperature in the vicinity of the bipolar transistor to be compensated.

In one embodiment, in order to ensure that the photon source is properlydirected toward the bipolar transistor, a reflective region is disposedupon an interior surface of the bipolar transistor package. At least aportion of the photons from the photon source are reflected onto thebipolar transistor to substantially maintain the junction potential at apredetermined level.

In an alternative embodiment, the photon source is located remotely fromthe bipolar transistor, with photons from the photon source beingdirected toward the bipolar transistor over a suitable optical medium,such as fiber optic cable. As is well-known in the art, a fiber opticcable is constructed from material that acts to contain optical energy,allowing the energy to be conducted from one point to another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of intrinsic carrier concentration versus temperature;

FIG. 2 illustrates temperature dependence of the band-gap voltage;

FIG. 3 depicts variation of junction potential with temperature fordifferent donor and acceptor concentrations;

FIG. 4 shows supplemental carrier concentration necessary to offsettemperature-induced changes in intrinsic carrier concentration;

FIG. 5 represents optical power required to maintain junction potentialat a predetermined value over temperature;

FIG. 6 is a schematic drawing of the optically compensated bipolartransistor of the present invention;

FIG. 7 shows the bipolar transistor, photon source, and temperaturesensor within a bipolar transistor package;

FIG. 8 illustrates the use of a photon source located remotely from thebipolar transistor; and

FIG. 9 is a flow chart of an algorithm for varying photon source opticalpower in response to temperature change.

DETAILED DESCRIPTION OF THE INVENTION

Emitter-base junction characteristics for bipolar transistors may beexpressed as: ##EQU1## where:

φ_(p) is the potential at the p side of the junction,

k₂ is Boltzmann's constant (in J/°K.),

T is temperature (°K.),

q is the elementary charge,

N_(a) is the acceptor carrier concentration,

n_(i) is the intrinsic carrier concentration,

and, ##EQU2## where:

φ_(n) is the potential at the n side of the junction, and

N_(d) is the donor carrier concentration.

φ, the sum of the potentials on either side of the junction, representsthe junction, or built-in, potential, and is expressed as: ##EQU3##

The intrinsic carrier concentration, n_(i), may be expanded explicitlyto show its temperature dependence:

    n.sub.i =CT.sup.3/2 e.sup.(-E.sbsp.g.sup./2k.sbsp.1.sup.T)

where:

E_(g) is the band-gap voltage,

k₁ is Boltzmann's constant (in eV/°K.), and ##EQU4## where:

m_(e) is the effective mass of the electron,

m_(h) is the effective mass of the hole,

M_(c) is the number of conduction band minima, and

h is Planck's constant (in J-s).

The expression for the constant C is derived from the representation ofintrinsic carrier concentration in terms of conduction band and valenceband state densities. Variation of intrinsic carrier concentration withtemperature, in accordance with the above-described relationship, isillustrated in FIG. 1.

Since n_(i), the intrinsic carrier concentration, can be written as:

    n.sub.i =(N.sub.c N.sub.v).sup.1/2 e.sup.(-E.sbsp.g.sup./2k.sbsp.1.sup.T)

and N_(c) and N_(v), the conduction band and valence band statedensities, respectively, can be expressed as: ##EQU5## where m_(de) isthe effective transverse mass of the electron, while m_(dh) is theeffective transverse mass of the hole. The effective transverse mass ofthe hole can be written as:

    m.sub.dh =(m.sub.hh.sup.*3/2 +m.sub.lh.sup.*3/2).sup.2/3

where m_(hh) *, the effective mass of the heavy hole, is 0.49, andm_(lh) *, the effective mass of the light hole, is 0.16. The effectivetransverse mass of the electron, m_(de), is 0.19.

The expression for φ, the junction potential, may now be rewritten as:##EQU6## Expanding the constant C, as previously defined, the expressionfor junction potential becomes: ##EQU7## If a new constant C₁ is nowdefined as follows: ##EQU8## then, ##EQU9##

The band-gap voltage, E_(g), is known to exhibit a temperaturedependency given by:

    E.sub.g =1.205-0.28×10.sup.-3 T

This temperature dependence of the band-gap voltage is graphicallydepicted in FIG. 2. For heavily-doped semiconductor materials, anadditional gap-narrowing factor is introduced: ##EQU10## where:

ε_(s) is the semiconductor permittivity, and

N_(e) is the emitter doping concentration.

This expression for the additional band-gap narrowing factor is easilysimplified to show its temperature relationship: ##EQU11##

Introducing the ΔE_(g) term into E_(g), and substituting, the equationfor junction potential becomes: ##EQU12## Then, rewriting thisexpression as: ##EQU13## and, finally: ##EQU14## clearly demonstratesthe temperature dependence of the junction potential, φ. FIG. 3illustrates the junction potential's temperature dependence fordifferent values of donor and acceptor carrier concentrations. It willbe observed that the junction potential is higher for more heavily dopedsemiconductor materials (upper curve).

Returning to the originally-derived expression for junction potential interms of donor and acceptor carrier concentrations, the junctionpotential at a predetermined reference temperature can be equated tojunction potential at an arbitrary temperature with the addition of anoptimizing carrier density term: ##EQU15## where:

T_(o) is the reference temperature (°K.),

n_(io) is the intrinsic carrier concentration at T_(o), and

n_(o) is the optimizing carrier density.

The final expression for junction potential can thus be solved forn_(o), the optimizing carrier density: ##EQU16## Therefore, asupplemental carrier concentration, n_(o), is necessary to maintain thejunction potential, φ, at a substantially constant level. The necessarylevels of supplemental carriers are plotted versus temperature in FIG.4, for different donor and acceptor concentrations.

It is known in the art that incident optical energy affects bulk carrierconcentration as follows: ##EQU17## where:

n is the bulk carrier concentration per unit volume due to incidentoptical energy,

P_(opt) is optimizing optical power,

v is optical frequency, and

τ is minority carrier lifetime.

Thus, n_(o), in carrier concentration per cubic centimeter, can beexpressed as: ##EQU18##

Substituting into the expression for optimizing carrier density yields:##EQU19## Now, solving for P_(opt) : ##EQU20## where P_(opt) is theoptical power, incident on the semiconductor surface, necessary tomaintain the junction potential at substantially the same level thatexists at a predetermined reference temperature (T_(o)). FIG. 5 is agraph of optical power versus temperature, for two different donor andacceptor concentrations, illustrating that the optical power required tomaintain junction potential at a predetermined level actually decreaseswith decreasing temperature.

FIG. 6 is a block diagram of an optically compensated bipolar transistorof the present invention. In the illustrated embodiment, a photon source(602) is provided from which photons (603) may be directed toward abipolar transistor (601) to be compensated. As depicted in the figure,the photon source (602) may be a light emitting diode (LED) or otherphoton source that emits photons of sufficient energy to create chargecarriers in semiconductor material. It is well known that optical outputpower of an LED may be controlled by varying its current.

Temperature in proximity to the bipolar transistor (601) is measured bya temperature sensor (605), which, in the preferred embodiment, is areference diode. Since junction potential of the reference diode (605)varies with temperature, sensing the junction potential of the referencediode provides an indication of temperature to a temperature sense andoptical power controller (604), which varies photon emission (603) inaccordance with temperature to maintain junction potential of thebipolar transistor at a predetermined level. Preferably, the referencediode (605) is optically passivated to render it substantially immune tochanges in junction potential introduced by impinging photons. Thus, thereference diode (605) remains a reliable indicator of temperatureregardless of the optical output power of the photon source (602).

Of course, the temperature sense and optical power controller (604) maybe implemented in a variety of ways well-known in the art. For example,one or more small-signal transistor amplifier stages could be arrangedwith the reference diode providing an input signal. In response to thereference diode input, current through the LED (602) could be controlledto vary LED output power.

The temperature sense and optical power controller (604) may also beimplemented in a microprocessor-based platform with an appropriate inputdevice, such as an analog-to-digital (A/D) converter, to detectreference diode voltage, and an appropriate output device to controlphoton source optical power.

An appropriate algorithm adaptable for execution in microprocessorsoftware is illustrated in FIG. 9. After program start (block 901),temperature is sensed in block 902. The temperature value is tested inblock 903 to determine if it has changed since the last measurement. Ifa change has occurred, optical output power of the photon source isadjusted accordingly in block 904 to maintain junction potential at apredetermined level. If no change is detected, processing resumes withanother temperature sense operation at block 902.

One method of directing photons toward a bipolar transistor to becompensated is shown in FIG. 7. As generally depicted by the number 700,a bipolar transistor package (701) is constructed to contain a substrate(703) having a bipolar transistor (707), a photon source (704), and areference diode (708). The package (701) has at least a partiallyreflective area (702) disposed upon an interior surface. The reflectivearea may be established, among other ways, by a metallization process.

In response to changes in temperature sensed by the reference diode(708), optical output power of the photon source (704) is adjusted sothat the photon source (704) emits varying numbers of photons (705). Atleast some of these photons (705) are reflected by the reflective area(702). These reflected photons (706) are directed onto the bipolartransistor (707) to substantially maintain junction potential at apredetermined level.

In an alternate embodiment, as illustrated in FIG. 8, a photon source(804) is located remotely from the bipolar transistor package (800). Thebipolar transistor (807) and reference diode (808), constructed on anappropriate substrate (803), are substantially housed within the package(800). An appropriate optical medium, such as a fiber optic cable (805),leads from the photon source (804) through the package boundary (801),so that photons (806) from the photon source (804) may be directedtoward the bipolar transistor (807).

What is claimed is:
 1. A method for compensating junction potential of abipolar transistor for changes caused by variation in intrinsic carrierdensity due to temperature change, the method comprising the stepsof:(a) sensing temperature of the bipolar transistor; and, in responsethereto (b) directing a sufficient number of photons toward the bipolartransistor to substantially maintain the junction potential at apredetermined level.
 2. The method in accordance with claim 1, whereinthe step (a) of measuring temperature comprises measuring junctionpotential of a reference diode.
 3. The method in accordance with claim2, wherein the reference diode is placed in relatively close proximityto the bipolar transistor.
 4. The method in accordance with claim 1,wherein the step (b) of directing a sufficient number of photons towardthe bipolar transistor comprises varying optical output power of aphoton source in response to the temperature sensed in step (a).
 5. Themethod in accordance with claim 4, wherein the photon source comprises alight emitting diode.
 6. A method for compensating junction potential ofa bipolar transistor for changes caused by variation in intrinsiccarrier density due to temperature change, the method comprising thesteps of:(a) providing a photon source having a variable output power,at least in part directed toward the bipolar transistor to becompensated; (b) providing a temperature sensor having an outputsubstantially related to temperature of the bipolar transistor to becompensated; and (c) adjusting the output power of the photon source inresponse to the temperature sensor output to substantially maintain thejunction potential of the bipolar transistor at a predetermined level.7. The method in accordance with claim 6, wherein the photon sourcecomprises a light emitting diode.
 8. The method in accordance with claim6, wherein the temperature sensor comprises a reference diode.
 9. Themethod in accordance with claim 8, wherein the reference diode isoptically passivated.
 10. An apparatus for compensating junctionpotential of a bipolar transistor for changes caused by variation inintrinsic carrier density due to temperature change, the apparatuscomprising:a photon source having a variable output power, at least inpart directed toward the bipolar transistor to be compensated; atemperature sensor having an output substantially related to temperatureof the bipolar transistor to be compensated; and means for adjusting theoutput power of the photon source in response to the temperature sensoroutput to substantially maintain the junction potential of the bipolartransistor at a predetermined level.
 11. The apparatus of claim 10,wherein the photon source comprises a light emitting diode.
 12. Theapparatus of claim 10, wherein the temperature sensor comprises areference diode.
 13. The apparatus of claim 12, wherein the referencediode is optically passivated.
 14. For a bipolar transistorsubstantially housed in a package having at least a partially reflectivearea disposed upon an interior surface thereof, a method forcompensating junction potential for changes caused by variation inintrinsic carrier density due to temperature change, the methodcomprising:(a) sensing temperature of the bipolar transistor; and, inresponse thereto (b) directing photons toward said at least partiallyreflective area so that at least a portion of the photons are reflectedonto the bipolar transistor to substantially maintain the junctionpotential at a predetermined level.
 15. For a bipolar transistorsubstantially housed in a package, a method for compensating junctionpotential for changes caused by variation in intrinsic carrier densitydue to temperature change, the method comprising:(a) sensing temperatureof the bipolar transistor; and, in response thereto (b) directingphotons toward said bipolar transistor from a photon source locatedremotely from said package to substantially maintain the junctionpotential at a predetermined level.
 16. The method in accordance withclaim 15, wherein photons from the remotely located photon source aredirected toward the bipolar transistor via a fiber optic cable.